Bottom anode Schottky diode structure and method

ABSTRACT

This invention discloses a bottom-anode Schottky (BAS) diode that includes an anode electrode disposed on a bottom surface of a semiconductor substrate. The bottom-anode Schottky diode further includes a sinker dopant region disposed at a depth in the semiconductor substrate extending substantially to the anode electrode disposed on the bottom surface of the semiconductor and the sinker dopant region covered by a buried Schottky barrier metal functioning as an Schottky anode. The BAS diode further includes a lateral cathode region extended laterally from a cathode electrode near a top surface of the semiconductor substrate opposite the Schottky barrier metal wherein the lateral cathode region doped with an opposite dopant from the sinker dopant region and interfacing the sinker dopant region whereby a current path is formed from the cathode electrode to the anode electrode through the lateral cathode region and the sinker dopant region in applying a forward bias voltage and the sinker dopant region depleting the cathode region in applying a reverse bias voltage for blocking a leakage current.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the Schottky diode devices. Moreparticularly, this invention relates to a device configuration andmethod of manufacturing Schottky diodes with a bottom-anode for multipleapplications.

2. Description of the Prior Art

Conventional Schottky diodes with a vertical structure that has thecathode disposed at the bottom of a substrate are limited by severaldifficulties of applications. As a high voltage bias is applied to thesubstrate, the vertical Schottky diodes with cathode at the bottom ofthe substrate are not compatible with some assembly configurations.Furthermore, for high voltage devices, such device configurationrequires an electrical isolation of the heat sink where the die ismounted, which causes limitations on heat dissipation and adds tocomplications to system designs when the vertical Schottky diodes areprovided with the cathode disposed at the bottom of a substrate.Different types of vertical Schottky diodes have been disclosed. FIG. 1Ashows a cross sectional view of a junction barrier Schottky diode formedon the top of a substrate and FIG. 1B shows an alternate Schottky diodeimplemented as a trench MOS-barrier Schottky diode on the top of asubstrate. In either of the Schottky diodes, the Schottky barrierregions are shielded off laterally by depletion of the PN junctions orthe MOS trenches when reverse bias reach a certain high voltage toreduce the leakage current. FIGS. 1C and 1D show alternate junctionbarrier Schottky (JBS) diodes in U.S. Pat. No. 4,134,123 with the P+regions pinching off the drain from the top anode region. However, theSchottky diodes with a vertical configuration and the cathode on thebottom as shown in these disclosures are still limited by thedifficulties discussed above for some practical applications, especiallywhen applied to portable devices that require small package with multifunctions to reduce component count and space. Specifically, for powerboost converter application as showed in FIG. 1E, the anode of anSchottky diode is connected to the Drain of a MOSFET, which is usuallyat the bottom of the MOSFET die. It is desirable to co-package theSchottky diode into the MOSFET package to reduce anode parasiticinductance; it is necessary to have two separate die pads for mountingthe MOSFET and the Schottky separately. This increases the assemblycomplexity and cost.

Therefore, a need still exists in the art of device design andmanufacture of Schottky diodes by leveraging existing Schottky contactmetallization schemes to provide new device configurations andmanufacturing method for new and improved Schottky diodes with anode onthe bottom of the substrate such that the above discussed problems andlimitations can be resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a new andimproved bottom-anode Schottky (BAS) diode implemented with a combinedanode-sinker connected to the bottom of the substrate and a buriedSchottky contact. The device configuration has a reduced cell pitch anda planar diode with an N-cathode channel interconnected between theanode-sinker region and a cathode contact. This structure is compatiblewith many foundries for reducing fabrication costs such that the abovediscussed technical difficulties and limitations can be overcome.

Specifically, it is an aspect of the present invention to provideimproved bottom-anode Schottky diode device for a boost converterapplication implemented with a combined anode-sinker connected through acathode region to a cathode contact along a lateral direction such thata bottom anode connection through the anode sinker significantly reducesthe anode inductance, and the use of a substrate source contactminimizes source resistance, and such that the Schottky diodes may besuitable for high gain and high frequency applications.

It is another aspect of the present invention to provide improvedbottom-anode Schottky diode device implemented with a combinedanode-sinker connected through a cathode region to a cathode contactalong a lateral direction such that the cell pitch can be reduced andthe specific on resistance (Rsp) is minimized for a given operatingvoltage to achieve low forward voltage drop.

It is another aspect of the present invention to provide improvedbottom-anode Schottky diode device implemented with a combinedanode-sinker connected through a cathode region to a cathode contactalong a lateral direction such that low reverse leakage and highbreakdown voltage are achieved.

It is another aspect of the present invention to provide improvedbottom-anode Schottky diode device implemented with a combinedanode-sinker connected through a cathode region to a cathode contactalong a lateral direction such that the BAS diodes are scalable and maybe compatible with high and low voltage applications.

It is another aspect of the present invention to provide improvedbottom-anode Schottky diode device implemented with a combinedanode-sinker connected through a cathode region to a cathode contactalong a lateral direction such that the BAS diode devices are ruggedbecause the possibility of latch-up is reduced with distributed bodycontact and reduced hot carrier injection and the peak voltage generatedaway from oxide layers.

It is another aspect of the present invention to provide improvedbottom-anode Schottky diode device implemented with a combinedanode-sinker connected through a cathode region to a cathode contactalong a lateral direction such that the BAS diode devices have lowleakage through a shielding scheme which extends under the Schottkycontact, to completely pinch-off the cathode-region and Schottky contactregion, at elevated cathode voltages.

Briefly in a preferred embodiment this invention discloses abottom-anode Schottky (BAS) diode that includes an anode electrodedisposed on a bottom surface of a semiconductor substrate. Thebottom-anode Schottky diode further includes an anode to substrateconnection formed by a sinker dopant region disposed at a depth in thesemiconductor substrate extending substantially to the anode electrodedisposed on the bottom surface of the semiconductor and the sinkerdopant region covered by a buried Schottky barrier metal functioning asan Schottky anode. The BAS diode further includes a lateral cathoderegion extended laterally from a cathode electrode near a top surface ofthe semiconductor substrate opposite the Schottky barrier metal whereinthe lateral cathode region doped with an opposite dopant from the sinkerdopant region and interfacing the sinker dopant region whereby a currentpath is formed between the cathode electrode and the anode electrodethrough the lateral cathode region and the sinker dopant regionconnected by a Schottky contact in applying a forward bias voltage andthe sinker dopant region depleting the cathode region in applying areverse bias voltage for blocking a leakage current. In an exemplaryembodiment, the semiconductor substrate is a P-type substrate and thesinker dopant region is a P-type doped region and the cathode region isa N-type doped region. In another exemplary embodiment, the cathoderegion laterally extended to the sinker dopant region having a gradeddopant profile near the buried Schottky barrier layer for providing areduced diode resistance. In another exemplary embodiment, the cathoderegion further includes a cathode contact dopant region doped with ahigher dopant concentration for contacting cathode barrier metalcovering the cathode contact dopant region. In another exemplaryembodiment, the BAS diode further includes a field oxide layer coveringthe cathode region insulating the buried Schottky barrier metal from thecathode region. In another exemplary embodiment, the BAS diode furtherincludes a field oxide layer covering the cathode region insulating theburied Schottky barrier metal from the cathode region. The buriedSchottky barrier metal extends over the top surface of the field oxideto function as a field plate. In another exemplary embodiment, the BASdiode further includes a field oxide layer covering the cathode regioninsulating the buried Schottky barrier metal from the cathode region;and the buried Schottky barrier metal aligned and cutoff at an endsurface of the field oxide layer whereby enabling a self-alignmentmanufacturing process. In another exemplary embodiment, the BAS diodefurther includes a field oxide layer covering the cathode regioninsulating the buried Schottky barrier metal from the cathode region anda BPSG passivation layer covering a buried Schottky barrier metal andthe field oxide layer. In another exemplary embodiment, the BAS diodefurther includes a cathode metal disposed on top of the BAS diodeelectrically contacting the cathode region to function as the cathodeelectrode. In another exemplary embodiment, the BAS diode furtherincludes a field oxide layer covering the cathode region insulating theburied Schottky barrier metal from the cathode region and a polysiliconfield plate covered with an oxide layer covering over the field oxidelayer. In another exemplary embodiment, the semiconductor substrate is aP-type substrate supporting a P-type epitaxial layer thereon the sinkerdopant region is a P-type doped region extending from the epitaxiallayer to the bottom surface of the P-type substrate.

Furthermore, this invention discloses a method for manufacturing aquasi-vertical diode on a semiconductor substrate that includes a topelectrode and a bottom-electrode. The method includes a step of forminga laterally extended doped region in the semiconductor substrate tofunction as a lateral current-path region for conducting a currentbetween the top and bottom electrodes of the diode. The method furtherincludes a step of forming a sinker doped-region at a depth in thesemiconductor substrate, interfacing with the lateral path region toextend vertically to a one of the top or bottom surface of the diode tofunction as a vertical current path. In another exemplary embodiment,the method further includes a step of electrically connecting thelateral current path region to one of the top or bottom electrodes. Inanother exemplary embodiment, the step of forming the sinker dopedregion at a depth in the semiconductor substrate includes a step offorming the sinker doped region as a vertical current path extending tothe bottom electrode, and the step of forming the lateral current pathregion further includes a step of connecting the laterally current pathregion to the top electrode. In another exemplary embodiment, the methodof manufacturing the quasi-vertical diode further includes a step ofmanufacturing a bottom-anode Schottky (BAS) diode by forming the bottomelectrode as a bottom anode electrode.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross sectional views for showing the typicaljunction barrier Schottky (JBS) diodes and trench MOS barrier Schottky(TMBS) of the conventional Schottky diodes.

FIGS. 1C and 1D are cross sectional views of Junction Barrier Schottkydiodes with top anode configuration.

FIG. 1E is a circuit diagram of a high voltage boost converter forportable application.

FIG. 2 is a cross sectional view of a bottom anode Schottky (BAS) diodeas a first embodiment of this invention.

FIGS. 3A to 3C are cross sectional views of alternate BAS diodes asalternate embodiments of this invention.

FIGS. 4A to 4J are a series of cross sectional views for describing themanufacturing processes to fabricate a BAS diode device of thisinvention.

FIG. 5A to 5L are serial cross sectional views for describing themanufacturing processes to fabricate another BAS diode device of thisinvention.

DETAILED DESCRIPTION OF THE METHOD

Referring to FIG. 2 for a cross sectional view of a bottom anodeSchottky (BAS) diode device of this invention. The BAS diode device issupported on a P++ substrate 105 functioning as a bottom anodeelectrode. A layer of P− epitaxial layer 110 is supported on top of thesubstrate 105. A deep sinker region 115 doped with P+ dopant is disposedbelow a Schottky barrier metal layer 120, forming an Ohmic contact withSchottky barrier metal 120 and vertically extending to the P++ substrate105 therefore electrically connecting the substrate bottom metal (notshown) to Schottky barrier metal 120. The Schottky barrier metal 120extends over a N region 125 forming a Schottky contact (diode) 122 atthe interface whereby the N region 125 functions as the Cathode and theSchottky metal layer 120 functions as the Anode of the Schottky diode.An anode to substrate connection is therefore established by the P+sinker contact electrically with the overlaying Schottky barrier metal120 and extending to the P+ substrate. The N region 125 may be gradeddoped to achieve low doping where the Schottky contact is locatedwithout increasing resistance. The deep sinker region 115 has anincreasing lateral dimension downwards at least to a depth that isdeeper than the depth of N region 125 and is further laterallycontacting the N-cathode region 125 that extends laterally with a N+cathode contact region 130 covered by an optional barrier metal 135 withfield oxide layer 140 covering over the N− cathode region between theSchottky contact 122 and the cathode contact region 130 thereforeinsulating the barrier metal 135 from the buried Schottky contact metal120. A BPSG passivation layer 145 covers the buried Schottky contactlayer 120 and the field oxide layer 140. A cathode metal 160 is formedon top of the barrier metal layer 135. Alternatively, the cathode metal160 may directly contact cathode contact region 130 without the barriermetal layer 135.

When the Schottky diode is forward bias, the bottom anode has a highervoltage and the cathode metal is applied with a lower voltage. Electronsflows from the cathode metal 160 to N+ cathode contact region 130 thenlaterally to N cathode region 125. The electrons go through the Schottkydiode 122 to Schottky metal 120 and then vertically down to the anodeelectrode (not shown) at the bottom of substrate through P+ sinkerregion 115. In reversed bias, the deep P+ sinker extending laterallyunder the Schottky contact pinch-off the N cathode region due to thedepletion of the PN junction where P+ sinker in contact with N cathoderegion therefore block the flow of leakage current. Compare to prior artJBS style diodes, where the p+ diffusions only provide lateralpinch-off, in the BAS structure of this invention, the pinch-off startsunder the Schottky contact due to the use of high energy deep implants.

In FIG. 2 the Schottky metal 120 overlaps a portion of field oxide layer140 forming an Schottky metal field plate that reduces the electricfield stress nearby the Schottky metal and field oxide interfacetherefore improve the device breakdown voltage. FIG. 3A shows analternate embodiment of a BAS diode similar to the BAS diode of FIG. 2except that the alternate BAS diode has a Schottky barrier layer 120self aligned to field oxide 140 therefore reduces one mask inmanufacturing process. In this case the layer of field oxide 140 may bereplaced by an oxide hard mask layer 140′ as shown in FIG. 3B. Inanother preferred embodiment as shown in FIG. 3C, a polysilicon fieldplate 150 covered with oxide layer 155 is formed over the field oxidelayer 140.

Referring to FIGS. 4A to 4J for a serial of side cross sectional viewsto illustrate the fabrication steps of a BAS diode device as that shownin FIG. 2. In FIG. 4A, a starting silicon substrate that includes a P++substrate 205 doped with Boron with a resistivity of 3 to 5 mOhm-cm or alower resistivity. The substrate 205 is preferably along acrystal-orientation of <100> as a standard prime. A P− epitaxial layer210 supported on the substrate 205 with a thickness ranging from 2 to 7micrometers typically doped with a low dosage of 5E14 to 5E15 for 20-60volts applications. In FIG. 4B, a pad oxide layer 212 is grown. A thickresist sinker mask (not shown) is applied as a first mask to carry outdeep sinker multiple energy implantations with an implanting dosageranging from 1E14 to 5E15 at energy of 180-360 keV. Additional sinkerimplants are performed with higher energy levels of 450-800 keV and800-1000 keV with an implanting dose in the range of 1E13 to 5E14 forexample to form the P+ sinker region 215. Formation of P+ sinker region215 by multiple energy implantations minimizes lateral diffusion,thereby reducing the pitch size. Also, the deeper the implant, thehigher the lateral straggle and distribution of the implant, to achievethe shielding under the Schottky contact. The implantation energydepends on the device's required breakdown voltage, which is a functionof the epi thickness since there is a need to try to maximize the dopingin the anode sinker contact region, away from the Schottky contact, tominimize the sinker resistance. Then the resist (not shown) is strippedfollowed by a sinker drive to thermally diffuse the sinker region 215.In FIG. 4C, a blanket N-cathode surface implant with arsenic orphosphorous ions is performed at a dose ranging from 5E11 to 1E13depending on the breakdown voltage and the thermal budge. In FIG. 4D, anitride layer 218 deposition is carried out to protect the N-cathoderegion 220 followed by applying an active mask to carry out a nitrideetch. A self-aligned N-drift implant is carried out optionally, tooptimize the dopant of cathode region 220 for reducing resistancewithout lowering breakdown voltage. The photo resist (not shown) isstripped and a field oxidation by applying a LOGOS process is carriedout to form the field oxide layer 225.

In FIG. 4E, the nitride 218 is stripped followed by an optionalsacrificial layer growth (228) and strip. An optional gate oxide layer(not shown) is grown (if there is also a transistor on the die). In FIG.4F, a short oxide etch is performed to remove the gate oxide layer toclear the active area. In FIG. 4G, an Schottky metal layer 230 isdeposited. The Schottky metal layer may compose of Ti, Co, Ta, etc. Arapid thermal anneal process (RTP) is carried out at 600-800° C. in anN₂ chamber for 20-60 seconds. An Schottky metal patterning process isperformed with an Schottky metal mask to etch a portion of the Schottkymetal 230 followed by stripping the resist (not shown). In FIG. 4H apassivation layer 235 composed of BPSG or USG is deposited followed by aBPSG reflow process. In FIG. 4I, a contact mask (not shown) is appliedto etch a contact opening through the passivation layer 235 and fieldoxide layer 225 followed by a contact implant to form a cathode contactregion 240. An optional anneal can be performed prior to metaldeposition, for example, an RTP step in the 800-900 C range, in N2, for10 to 60 seconds. In FIG. 4J, a cathode metallization process isperformed to form cathode metal with options of the barrier metal layer245 and/or Tungsten plug (not shown). An aluminum layer 250 is formed asthe cathode metal over the top surface. A passivation layer (not shown)may be formed to protect the device.

Referring to FIGS. 5A to 5J for a serial of side cross sectional viewsto illustrate and alternate set of fabrication steps of a BAS diodedevice as that shown in FIG. 3. In FIG. 5A, a starting silicon substratethat includes a P++ substrate 305 doped with Boron with a resistivity of3 to 5 mOhm-cm or a lower resistivity. The substrate 305 is preferablyalong a <100>-crystal orientation as a standard prime. A P− epitaxiallayer 310 supported on the substrate 305 with a thickness ranging from 2to 7 micrometers typically doped with a low dosage of 5E14 to 5E15 for20-60 volts applications. In FIG. 5B, a pad oxide layer 312 is grown. Athick resist sinker mask (not shown) is applied as a first mask to carryout deep sinker multiple energy implantations with an implanting dosageranging from 1E14 to 5E15 at energy of 180-360 keV. Additional sinkerimplants are performed with higher energy levels of 450-800 keV and800-1000 keV with an implanting dose of 1E14 to form the sinker P+sinker region 315. Formation of P+ sinker region 315 by multiple energyimplantations minimizes lateral diffusion thereby reducing the pitchsize. The implantation energy depends on the device's required breakdownvoltage, which is a function of the EPI thickness. Then the resist (notshown) is stripped followed by a sinker drive to thermally diffuse thesinker region 315. In FIG. 5C, a blanket N-cathode surface implant witharsenic or phosphorous ions is performed at a dose ranging from 5E11 to1E13 depending on the breakdown voltage and the thermal budget. In FIG.5D, a nitride deposition is carried out to protect the N-cathode region320 followed by applying an active mask to carry out a nitride etch. Aself-aligned N-drift implant is carried out optionally to optimizecathode region 320. The photo resist (not shown) is stripped and a fieldoxidation by applying a LOCOS process is carried out to form the fieldoxide layer 325. The oxidation LOCOS process can be optionally appliedto diffuse the N-cathode region 320. The field oxide layer 225 may begrown to be approximately 0.3 to 1.0 μm thick—preferably about 0.5 μm ata temperature of 900 to 1050° C.

In FIG. 5E, the pad oxide 312 is stripped followed by an optionalsacrificial layer growth (328) and strip. An optional gate oxide layer(not shown) is grown followed by depositing a polysilicon layer orpreferably a polycide layer having a thickness up to 2000 Angstroms (notshown). In FIG. 5F, a short oxide etch is performed to remove the gateoxide layer to clear the active area. In FIG. 5G, an Schottky metallayer 330 is deposited. The Schottky metal layer may compose of Ti, Co,Ta, W, Mo, etc. A rapid thermal anneal process (RTP) is carried out at600-800° C. in N₂ chamber for 20-60 seconds. A Schottky metal patterningprocess is performed with a salicide (self-aligned silicide) etch toremove the portions of the Schottky metal layer 330 which did not reactwith underlying silicon to form silicide, thus forming a self-alignedanode to substrate connection adjacent to a Schottky contact. The etchprocess can be a wet etch, using ammonium hydroxide, peroxide and watersolution for example. Following the Salicide etch process, an optionalRTP in the 800-900 C range, in N2, for 10 to 60 seconds, may be applied.In FIG. 5H a passivation layer 335 composed of BPSG or USG is depositedfollowed by a BPSG reflow process. In FIG. 5I, a contact mask (notshown) is applied to etch a contact opening through the passivationlayer 335 and the field oxide layer 325 followed by a contact implant toform a cathode contact region 340. An RTP anneal to activate the contactimplant can also be used. In FIG. 5J, a cathode metallization process isperformed to form the cathode metal with options of barrier metal layer345 and or Tungsten plug (not shown). Then an aluminum layer 350 isformed as the cathode metal over the top surface. A passivation layer(not shown) is formed to protect the device. FIG. 5K shows an alternateembodiment of the device that the field oxide 325 is replaced by a padoxide where the process of growing field oxide in FIG. 5D is replaced byLPV deposition then patterned. FIG. 5L shows another embodiment with apolysilicon field plate 360 formed before Schottky metal layer 330 isdeposited. A polysilicon layer 360 is deposited and pattern with a maskthen oxidized to form the field plate 360 with oxide layer 365. Thedevice then continues the rest of process to form an Schottky diode witha floating polysilicon field plate that improves the breakdown ofdevice.

According to above processing steps for manufacturing the bottom anodeSchottky diodes, various design parameters may be adjusted for differentapplications with optimal performances. The width of the top contact tothe P+ anode-sinker may be minimized to achieve a benefit of reducingthe cell pitch. The width of the top surface overlapping the N-cathoderegion for Schottky contact may be maximized to achieve a benefit ofmaximizing Schottky contact to reduce resistance and increase currenthandling capability. The depth and lateral diffusion under the N-cathoderegion of the deep anode-sinker region may be maximized and The dopingconcentration of the N-cathode and the overlapping deep anode sinkerregion may be optimized to maximize the pinch off at a high voltage butwith a maximized Schottky contact region defined by the overlap of theSchottky metal and the N− region.

By adjusting various design parameters as described above, furtheranalyses confirm that the BAS diode devices according to the disclosuresmade in this invention can achieve a cell pitch in a range ofapproximately 7 μm for applying to device operated at 35-45 volts andabout 9 μm for a device operated around 45-65 volts. A high breakdownvoltage is achievable even if a quasi-vertical structure is implementedwith a portion of the current-flow path is along a lateral direction.Specifically, a breakdown voltage in the range of 40-65 volts isachievable at a current of approximately 10 μA and at room temperature.Higher breakdown voltage is achievable with higher anode-sinker dopingconcentration or with a lower N-cathode blank implant dosage thatconfirms a pinch off function takes place in the deeper anode-sinkerregion. A low forward voltage is achievable. Specifically, the forwardvoltage is approximately 0.7 volts at 100 A/cm², i.e., 1 A in an area of1.0 mm² in the active area and the forward voltage is approximately 1.1volts at 1000 A/cm², i.e., 10 A in an area of 1.0 mm² in the activearea. Compared with a silicon control diode, the forward voltagesresulting from these analyses confirm that the bottom-diode structure isindeed functioning as a true Schottky diode.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

1. A bottom-anode Schottky (BAS) diode comprising an anode electrodedisposed in a semiconductor substrate near a bottom surface of saidsemiconductor substrate, said bottom-anode Schottky diode furthercomprising: a sinker dopant region disposed at a depth in saidsemiconductor substrate extending substantially to said anode electrodedisposed near said bottom surface of said semiconductor substrate andsaid sinker dopant region covered by a buried Schottky barrier metalfunctioning as an Schottky anode; and said BAS diode further comprisinga lateral cathode region extended laterally from a cathode electrodenear a top surface of said semiconductor substrate opposite saidSchottky barrier metal wherein said lateral cathode region doped with anopposite dopant from said sinker dopant region and interfacing saidsinker dopant region whereby a current path is formed between saidcathode electrode and said anode electrode through said lateral cathoderegion and said sinker dopant region in applying a forward bias voltageand said sinker dopant region depleting said cathode region in applyinga reverse bias voltage for blocking a leakage current.
 2. The BAS diodeof claim 1 wherein: said semiconductor substrate is a P-type substrateand said sinker dopant region is a P-type doped region and said cathoderegion is a N-type doped region.
 3. The BAS diode of claim 1 wherein:said cathode region laterally extended to said sinker dopant regionhaving a graded dopant profile near said buried Schottky barrier layerfor providing a reduced diode resistance.
 4. The BAS diode of claim 1wherein: said cathode region further comprising a cathode contact dopantregion doped with a higher dopant concentration for contacting cathodebarrier metal covering said cathode contact dopant region.
 5. The BASdiode of claim 4 further comprising: an oxide layer covering saidcathode region insulating said buried Schottky barrier metal from saidcathode contact dopant region.
 6. The BAS diode of claim 4 furthercomprising: a field oxide layer covering said cathode region insulatingsaid buried Schottky barrier metal from said cathode contact dopantregion; and said buried Schottky barrier metal extending over said a topsurface of said field oxide to function as a field plate.
 7. The BASdiode of claim 4 further comprising: an oxide layer covering saidcathode region insulating said buried Schottky barrier metal from saidcathode contact dopant region; and said buried Schottky barrier metalself-aligned and cutoff at an end surface of said oxide layer.
 8. TheBAS diode of claim 4 further comprising: an oxide layer covering saidcathode region insulating said buried Schottky barrier metal from saidcathode contact dopant region and a BPSG passivation layer covering aburied Schottky barrier metal and said oxide layer.
 9. The BAS diode ofclaim 8 further comprising: A cathode metal disposed on top of said BASdiode electrically contacting said cathode region to function as saidcathode electrode.
 10. The BAS diode of claim 4 further comprising: afield oxide layer covering said cathode region insulating said buriedSchottky barrier metal from said cathode contact dopant region and apolysilicon field plate covered with an oxide layer covering over saidfield oxide layer.
 11. The BAS diode of claim 1 wherein: saidsemiconductor substrate is a P-type substrate supporting a P-typeepitaxial layer thereon said sinker dopant region is a P-type dopedregion extending from said epitaxial layer to said P-type substrate.